An optical scanning device used in an image forming apparatus such as a color printer includes incident optical systems for four colors of YCMBk including light sources for emitting light beams, a polygon mirror for reflecting the light beams incident thereon from these incident optical systems and scanning the peripheral surfaces of photoconductive drums and scanning lenses for imaging the light beams on the peripheral surfaces. In an optical scanning device required to be compact and low in cost, one polygon mirror is commonly used for light beams of four colors. In such an optical scanning device, a counter scanning method is employed in which two scanning lenses each corresponding to the incident optical systems of two colors are arranged across a polygon mirror. In this case, light beams of two colors are incident on one deflection surface of the polygon mirror and light beams of the remaining two colors are incident on another deflection surface.
An optical scanning device of a counter scanning type is disclosed in Publication of Japanese Patent No. 3487482 in which incident optical systems are arranged such that an angle formed by an optical axis of a light beam incident on a deflection surface of a polygon mirror and an optical axis of the light beam propagating from this deflection surface toward the peripheral surface of a photoconductive drum (hereinafter, referred to as an “incident open angle”) differs in each of incident optical systems of two colors. However, in the optical scanning device having such a configuration, the incident open angle changes if an optical housing housing optical components is thermally deformed due to an environmental variation such as an internal temperature increase caused by the continuous operation of an image forming apparatus. If the incident open angle changes, a reflection point on the polygon mirror with respect to the scanning lens varies, with the result that the deterioration of imaging performance of the scanning lens on the peripheral surface of the photoconductive drum is caused. If imaging performance is deteriorated in this way, a color shift occurs in a formed color image, leading to a quality reduction of the color image.
Among optical scanning devices configured such that light beams of two colors are caused to be incident on one deflection surface of a polygon mirror at different incident open angles, Japanese Unexamined Patent Publication No. 2008-122706 discloses an optical scanning device using scanning lenses formed by vertically overlapping two lenses in such a manner as to be shifted from each other in a main scanning direction. The shapes of the two scanning lenses facing each other across the polygon mirror are in a relationship mirror-inverted with respect to each other. Specifically, in each of the two scanning lenses, a separating direction of a lens center of the upper lens from that of the lower lens is the same. In the optical scanning device having such a configuration, the deterioration of imaging performing of the scanning lenses on the peripheral surfaces of the photoconductive drums can be suppressed, but two scanning lenses having different shapes are used. Thus, common use of components cannot be achieved for two scanning lenses facing each other across the polygon mirror and, along with that, a reduction of component production man-hours also cannot be achieved.